The invention relates to vulcanizable elastomer compositions and in particular to vulcanizable diene rubber compositions intended for tire production. More particularly, the invention concerns novel anti-reversion agents capable of being used in such compositions.
The vulcanization of unsaturated elastomers in the presence of sulphur or sulphur donors results in a change into the elastic state of the elastomer mixture which is accompanied by modification of the mechanical properties of the mixture. This vulcanization proceeds during curing of the mixture by the formation of a network of sulphur bridges between the different polymer molecules.
The network formed in this manner nevertheless suffers from thermal instability. Heating of the tires during travel generates thermal stresses in the cured elastomer mixtures constituting the network. These thermal stresses have an overcuring effect which, when the temperature of the mixture reaches a value approaching the vulcanization temperature, brings about the phenomenon of reversion. Reversion is a breakdown of the elastomer network due to the destruction or shortening of the polysulphurized crosslinking bridges. This breakdown of the network results in a drop in modulus and unfavorably modifies the mechanical properties of the elastomer mixture.
An attempt has accordingly been made to limit or even eliminate this phenomenon of reversion by adding to the rubber compositions specific additives known as anti-reversion agents which make it possible to stabilise the vulcanized rubber compositions thermally.
Bismaleimides constitute one commonly described family of anti-reversion agents. Used alone, these compounds are in particular illustrated in the document JP91/056544. Combinations of bismaleimides and other compounds, such as other anti-reversion agents or vulcanization accelerators, are in particular the subject matter of documents EP 0 191 931, U.S. Pat. Nos. 5,328,636, 5,616,279, 5,736,611 or alternatively EP 0 823 453.
Bis-citraconimides have also been described in anti-reversion agent applications, alone or in combination with other additives, in particular in documents WO 92/07904, U.S. Pat. No. 5,623,007 and U.S. Pat. No. 5,872,188. Bis-citraconic acid and the derivatives thereof are also mentioned as an agent which improves reversion resistance, in particular in document U.S. Pat. No. 5,696,188.
Bis-succinimides constitute another class of anti-reversion agents and the derivatives thereof, which are described in particular in documents U.S. Pat. No. 5,844,049 and WO 96/20246.
Document JP 94/256573 describes the use of dithiocarbamate salts for improving reversion resistance. Document EP 0 832 920 discloses the use of zinc dithiophosphate for improving the thermal stabilisation of vulcanized rubber compositions. Document U.S. Pat. No. 4,417,012 describes poly-thiosulphate or poly-thiosulphonate compounds which also make it possible to stabilise vulcanized rubber compositions thermally against the phenomenon of reversion. More recently, U.S. Pat. No. 5,736,611 teaches improvement of the reversion resistance of a rubber composition comprising polyacrylates.
Anti-reversion agents are a major focus of research, particularly in the field of tires, where optimum thermal stability is fundamental. Tires cannot in fact withstand substantial modification of their mechanical properties without such modification impairing their performance.